Linking Western Tethyan Rhynchonellide morphogroups to the key post-Palaeozoic extinction and turnover events

Abstract

Genetic control undoubtedly plays the most decisive role in the development of characters in rhynchonellides, especially in those structural features such as shell microstructure and brachidial architecture. However, considering the relationship between shell microstructure and type of crura in post-Palaeozoic rhynchonellides supported by recent growing research, the present study links this striking matching to the distribution of rhynchonellide morphogroups around the main post-Palaeozoic mass extinction and diversification events, innovatively introducing the ribbing pattern as a feature widely argued as indicator of deeper/shallower habitats and particular ecological conditions. The analyses performed on the Western Tethys rhynchonellide database and, directly on 28 species from the peri-Iberian margins around selected post-Palaeozoic extinction/diversification episodes, reveal a recurrent distribution pattern according to the habitat considered (epicontinental vs. epioceanic). The smooth, eurinoid, septifal/arcual body plan is the distinctive inhabitant of deeper epioceanic environments, also being the last to withdraw from the epicontinental platforms when the environmental conditions were adverse, and the pioneers or opportunistic taxa conducting repopulation of these epicontinental habitats when the conditions were re-established after crises. The ribbed, eurinoid, falciform morphogroup seems to be the more resilient stock on epicontinental platforms in relation to ecological crises. Conversely, the ribbed, leptinoid, raducal group is typical inhabitant of epicontinental seas when normal conditions prevail and shows an uneven distribution unlinked to critical events, suggesting a more specialist pattern with a diversification related with a secondary differentiation. It is suggested that this distribution dynamic of morphogroups was controlled by the availability of requirements to generate each type of crura and microstructure in epicontinental/epioceanic habitats, such as metabolic or oxygen restrictions, or the filter-feeding efficiency of the lophophore. A research line starting-point is thus herein introduced which can lead to shed light into the adaptive strategies of rhynchonellides, relating particular morphotypes with potential triggering factors of mass extinction events.